Gen. James Thurman, the commander of the U.S. Forces Korea, expressed worries on Thursday over North Korea's threat to scrap the armistice that halted the 1950-1953 Korean War.
"It concerns me when any signatory to a mutual agreement makes a public statement contrary to that agreement," he told reporters.
The North had earlier threatened to turn Seoul and Washington into a "sea of fire" and "wave its flag" on the ridge of Mt. Halla on Jeju Island.
Thurman apparently decided to issue a statement of his own following the lead of the South Korean Joint Chiefs of Staff, who warned the previous day they will "strongly and sternly" strike the "starting point, its supporting forces and command" if the North attempts a provocation.
Thurman said, "We remain ready to defend the Republic of Korea."
A military source said the statement "sends a warning to the North that if it provokes, South Korea and the U.S. will respond together."
On Thursday, when the UN was about to impose tougher sanctions against North Korea over its latest nuclear test, the official Rodong Sinmun daily threatened to attack the U.S. "We have no binding agreement with the U.S. whereby either side can find fault with the other if one presses the nuclear button first," the paper said. The "means of strike" are now on standby, it claimed, and "if the button is pressed, it will be launched, and the bastion of aggression will turn into a sea of fire."